A. The Fusion Process
The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. See, for example, Varshneya, Arun K., “Flat Glass,” Fundamentals of Inorganic Glasses, Academic Press, Inc., Boston, 1994, Chapter 20, Section 4.2., 534-540. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of liquid crystal displays (LCDs).
The fusion process, specifically, the overflow downdraw fusion process, is the subject of commonly assigned U.S. Pat. Nos. 3,338,696 and 3,682,609, to Stuart M. Dockerty, the contents of which are incorporated herein by reference. A schematic drawing of the process of these patents is shown In FIG. 1. As illustrated therein, the system includes a supply pipe 9 which provides molten glass to a collection trough 11 formed in a refractory body 13 known as an “isopipe.”
Once steady state operation has been achieved, molten glass passes from the supply pipe to the trough and then overflows the top of the trough on both sides, thus forming two sheets of glass that flow downward and then inward along the outer surfaces of the isopipe. The two sheets meet at the bottom or root 15 of the isopipe, where they fuse together into a single sheet. The single sheet is then fed to drawing equipment (represented schematically by arrows 17), which controls the thickness of the sheet by the rate at which the sheet is drawn away from the root. The drawing equipment is located well downstream of the root so that the single sheet has cooled and become rigid before coming into contact with the equipment.
As can be seen in FIG. 1, the outer surfaces of the final glass sheet do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces only see the ambient atmosphere. The inner surfaces of the two half sheets which form the final sheet do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.
As is evident from the foregoing, isopipe 13 is critical to the success of the fusion process. In particular, the dimensional stability of the isopipe is of great importance since changes in isopipe geometry affect the overall success of the process. See, for example, Overman, U.S. Pat. No. 3,437,470, and Japanese Patent Publication No. 11-246230.
Significantly, the conditions under which the isopipe is used make it susceptible to dimensional changes. Thus, the isopipe must operate at elevated temperatures on the order of 1000° C. and above. Moreover, in the case of the overflow downdraw fusion process, the isopipe must operate at these elevated temperatures while supporting its own weight as well as the weight of the molten glass overflowing its sides and in trough 11, and at least some tensional force that is transferred back to the isopipe through the fused glass as it is being drawn. Depending on the width of the glass sheets that are to be produced, the isopipe can have an unsupported length of 1.5 meters or more.
To withstand these demanding conditions, isopipes 13 have been manufactured from isostatically pressed blocks of refractory material (hence the name “iso-pipe”). In particular, isostatically pressed zircon refractories have been used to form isopipes for the fusion process. As known in the art, zircon refractories are materials composed primarily of ZrO2 and SiO2, e.g., in such materials, ZrO2 and SiO2 together comprise at least 95 wt % of the material, with the theoretical composition of the material being ZrO2.SiO2 or, equivalently, ZrSiO4. Even with such high performance materials, in practice, isopipes exhibit dimensional changes which limit their useful life. In particular, isopipes exhibit sag such that the middle of the unsupported length of the pipe drops below its outer supported ends. The present invention is concerned with controlling such sag.
A primary contributor to the sag of an isopipe is the creep rate {dot over (ε)}=dε/dt of the material from which it is made. As known in the art, for many materials, creep rate as a function of applied stress σ can be modeled by a power law expression of the following form:{dot over (ε)}=Aσnexp(Q/T)  (1)where T is temperature and A, n, and Q are material dependent constants. See Kingery et al., “Plastic Deformation, Viscous Flow, and Creep,” Introduction to Ceramics, 2nd edition, John Wiley & Sons, New York, 1976, 704-767 and, in particular, equation 14.9. Being the time derivative of strain, the units of creep rate are length/length/time. Because in equation (1) creep rate varies as stress raised to a power, i.e., σn, the use of equation (1) will be referred to herein as the “power law model.”
Lowering the creep rate of the material used to make an isopipe results in less sag during use. As discussed in detail below, in accordance with certain aspects of the invention it has been found that the sag of an isopipe can be reduced by forming the isopipe from an isostatically pressed zircon refractory having a titania (TiO2) content which is greater than 0.2 wt % and less than 0.4 wt %, e.g., a TiO2 content of approximately 0.3 wt %. In particular, it has been found that such a zircon refractory exhibits a lower mean creep rate than zircon refractories used in the past to from isopipes and having a titania content of about 0.1 wt %.
In addition, it has also been found that controlling the titania content of a zircon refractory to be within the above range significantly enhances the usefulness of the power law model of equation (1) in modeling the sag of isopipes during use. This enhanced usefulness results from improved 95% confidence intervals for the mean creep rates predicted by the model when equation (1) is evaluated for a particular set of σ, T values. Such improved 95% confidence intervals, in turn, mean that the sag which an isopipe will exhibit during use can be more accurately modeled using, for example, a finite element or other modeling technique. More accurate modeling greatly enhances the ability to develop improved isopipe designs since numerous designs can be evaluated theoretically with only the best candidates being selected for actual construction and testing.
B. Zircon Refractories
As indicated above, the present invention relates to isopipes composed of a zircon refractory having a titania concentration within specified limits. Corhart Refractories Corporation (Louisville, Ky.) offers a number of zircon refractories containing varying amounts of TiO2. For example, Corhart's ZS-835 product is specified to contain 0.2 wt % TiO2, its ZS-835HD product 0.4 wt %, its Zircon 20 product 0.7 wt %, and its ZS-1300 product 1.2 wt %.
As a raw material, zircon can have varying amounts of titania. For example, U.S. Pat. No. 2,752,259 reports that the zircon used in its examples had 0.34 wt % TiO2, while the zircon used in U.S. Pat. No. 3,285,757 had 0.29 wt % TiO2. U.S. Pat. Nos. 3,347,687 and 3,359,124 each describe zircons having TiO2 concentrations of 0.2 wt %. In addition to being naturally present in zircon as a raw material, TiO2 can also be a component of clays used in producing zircon refractories. See U.S. Pat. Nos. 2,746,874 and 3,359,124.
Other discussions of the use of titania in zircon products can be found in Goerenz et al., U.S. Pat. No. 5,407,873 which discloses (1) the use of phosphorus compounds to improve the corrosion resistance of zirconium silicate bricks and (2) the use of titanium dioxide as a sintering aid in the manufacture of such bricks. Although the patent states that sintering can be improved by adding between 0.1 wt % and 5 wt % of titanium dioxide, all of the examples of the patent use more than 1 wt % of titanium dioxide and the patent's preferred composition consists of 98 wt % zirconium silicate, 1.5 wt % titanium dioxide, and 0.5 wt % of a phosphorous compound.
Wehrenberg et al., U.S. Pat. No. 5,124,287 relates to the use of zirconia in particle form to improve the thermal shock resistance of zircon refractories. Titania is employed to enhance grain growth during sintering. The patent claims titania concentrations between 0.1 wt % and 4 wt %. The preferred titania concentration is 1 wt %, and when blistering is a problem, only 0.1 wt % titania is used. The patent states that “grog” having a titania concentration of 0.2 wt % was used as a starting material for some of its examples.
Significantly, none of the foregoing disclosures regarding the use of titania in zircons relates to employing titania concentration as a means to control the creep rate of a zircon refractory, or to enhance the ability of a power law model to represent the material, or to achieve the ultimate goal of reducing the sag of an isopipe made of a zircon refractory.